1. Field of the Invention
The present invention relates generally to switching power regulator circuits, and more specifically, to a resonant switching power converter in which dead time between pulses is adaptively controlled.
2. Background of the Invention
In resonant switching power converters, to avoid introducing losses and stresses in the switching circuit, it is desirable to switch the transistors in the switching circuit when the voltage difference across the source and drain is at a minimum. Since any potential present across a switching transistor and any drain-source connected capacitor at the time of the transistor's activation will result in a waste of energy, zero-voltage switching (ZVS) control is desirable. If the switching circuit switches too early, the transistor and capacitors will be discharged through the switching circuit, wasting energy. However, if the switching circuit is switched too late, energy can be wasted by currents conducted through the body diodes of the transistors back to the power supply rail. ZVS control raises the efficiency of the power supply and also reduces the stresses experienced by the switching transistor, increasing reliability. Further, transients generated at the switching frequency can cause electromagnetic interference (EMI) and audible noise, as the switching frequency of such resonant converters is typically within the audio range.
In order to provide ZVS control, the dead time between pulses may be set to a time duration such that the input to the resonant tank has swung from near one power supply rail to the other power supply rail (for bipolar pulses), or has completed a full cycle (for unipolar pulses). However, since the frequency of the resonant converter is varied in order to control the inductor current and thus the energy supplied to the load, the time duration corresponding to the dead time will not be correct for all operating conditions. Typically the dead time is set to favor the higher power operating condition (i.e., the higher frequency operating condition), since losses due to non-optimum switching times are greater for higher inductor current levels.
Therefore, it would be desirable to provide a resonant switching power converter having improved efficiency, reduced stresses and audio/EMI noise.